Estrogens function in the development of female sex characteristics and play key roles in the hormone-regulated diseases. The biochemical actions of steroid hormones are mediated through their specific receptors. To understand the regulatory mechanism of gene expression of the estrogen receptor, it is important to know the DNA sequences of the estrogen receptor gene. Our primary research aims are to determine the DNA sequences of the control regions (such as RNA polymerase and mRNA ribosomal binding sites), the structural gene, the splicing junctions of the estrogen receptor gene of chicken oviduct. We plan to purify the nuclear estrogen receptor to homogeneity and determine part of the amino acid sequences. Based on the amino acid sequences, oligodeoxynucleotides are synthesized to use as a probe for screening of recombinant plasmids containing estrogen receptor cDNA. The chromosomal DNA library is cloned with Lambda phage DNA, and the estrogen receptor gene clones are selected with the specific cDNA segment obtained from the recombinant plasmids. Restriction enzyme mapping and DNA sequencing are done to analyze the estrogen receptor gene. Immunochemical similarities among estrogen receptors suggest that the estrogen receptor gene sequences of humans can be worked out by using the cloned estrogen receptor cDNA of chicken (or calf) as the screening probe. The cloned estrogen receptor cDNA could be employed to produce a large amount of estrogen receptor for physical and chemical studies.